Specific Aims: 1. To assess whether the use of needle exchange programs (NEPs) by injection drug users (IDUs) is associated with increased or decreased odds of human immunodeficiency virus (HIV) seroconversion; and 2. To assess whether IDUs use of bleach is associated with increased or decreased odds of HIV seroconversion. Design: Case-control study, nested within five ongoing NIDA-funded prospective studies of out-of-treatment IDUs in cities with NEPs. The Principal Investigators of those studies have agreed to participate in the proposed study. Subjects: Cases: IDUs in the five prospective studies who develop HIV antibodies; Controls: For each index case, a random sample of two IDUs who remain HIV seronegative. The controls will be identified from the same study and study period as the index case. Measurements: l. Predictor variables: Use of the NEP will be assessed by adding three questions to the questionnaire currently in use in the NIDA Cooperative Agreement studies; the questionnaire already inquires about bleach use; 2. Outcome variable: HIV seroconversion, already being assessed by serial blood draws in the prospective studies; 3. Confounding variables: Potential confounders such as age, ethnicity, gender, and injection and sex HIV risk behaviors will be assessed using the current NIDA Cooperative Agreement questionnaire. Statistical Issues: Potential confounders will be controlled by using logistic regression. If 30% of HIV seroconverters have ever used NEPs (or bleach), at alpha = 0.05 aid beta = 0.2, 150 seroconverters would be needed to detect an odds ratio of 0.56. We predict approximately 149 seroconverters (with 298 controls) during the proposed three-year study period. Conclusion: This proposal uses data already being collected in NIDA-funded prospective studies to assess whether NEPs and bleach are associated with changes in the odds of HIV seroconversion among IDUs.